If you thought Pirates of the Caribbean‘s Capt. Jack Sparrow was strange, just wait until you see Johnny Depp’s next movie. As Sweeney Todd, in Tim Burton’s adaptation of the Stephen Sondheim musical (opening Dec. 21), the actor summons all sorts of dark energy to play a singing, murderous London barber. ”He makes Sid Vicious look like the innocent paper boy,” Depp says. ”He’s beyond dark. He’s already dead. He’s been dead for years.” EW caught up with the star to talk about the role, what it was like performing opposite Helena Bonham Carter and Sacha Baron Cohen, why he hates watching himself on screen, and — aaaargh! — how it feels to be an attraction at Disneyland.

ENTERTAINMENT WEEKLY: This Sweeney dude — he is messed up! You are going to freak out a lot of pre-pubescent girls with this character.JOHNNY DEPP: Ah, finally! It’s a radical left turn, that’s for sure. The difficulty and the challenge [was] taking a character like that and attempting to make people feel for him, at the same time that he’s slashing people up. Not easy. But I certainly hope it came across that way.

Musical lovers and Stephen Sondheim fanatics know Sweeney Todd really well. What about the general public?Somebody sent me this thing from online. Somebody said, after they saw the trailer, ”I don’t understand why in the middle of that trailer Depp broke into a song.” Like, ”Whoa — What is he doing?”

Singers say Sondheim’s melodies can be incredibly tough. Why?It’s real obtuse stuff. When you start to take those pieces apart, melody line by melody line, it’s a lot of half-steps, which is not real easy to do. Kind of go G to A-flat to A to B-flat. It’s super, ultra complicated, these notes that shouldn’t work together at times. But he made them so.

Did Sondheim have any good advice for you?He said to me early on, it was much more about the acting work than the singing. He felt the singing was secondary to hitting the notes emotionally. I didn’t believe him. [Laughs] I think he was probably saying that to make me feel better about what I was about to attempt.

And what did that feel like?Frightening. Really frightening! When Tim asked if I’d be into it, he said, ”Do you think you can sing?” And I said, ”Honestly, I don’t know.” I’m not tone deaf, so I knew I could stay in key to some degree. But I didn’t know if I could sustain a note, or belt one out.

You were in a number of rock and roll bands before you became an actor. Didn’t you do any singing in those?Virtually none. Just backup.

And yet Sondheim approved you without an audition.Sondheim, bless him, had barely heard me talk. So when he said, ”He’ll be fine,” it was a real shock.

What did Tim Burton say to you after he finally heard your singing voice?He couldn’t have been sweeter about it. He was really supportive, and said he really liked it. It was the reaction I was praying for.

NEXT PAGE: ”Singing couldn’t be more foreign to me in a lot of ways, but at the same time, I need to incorporate my own process to find it, to see where I land.”